1. FIeld of the Invention
This invention pertains to large diameter fluid flow ducts for use in ocean engineering and the like. More particularly, it pertains to a connection of the upper end of the pipe to a floating structure, which connection is so arranged for ready and simple connection and release of the pipe assembly.
2. Review of the Prior Art and Its Problems
Many sophisticated proposals have been made in the field of ocean engineering which call for the use of large diameter vertical ducts of great length extending from at or near the ocean surface to lower ends unconnected to the ocean floor. These proposals include concepts for ocean thermal energy conversion and for mariculture.
The ocean thermal energy conversion (OTEC) concepts propose to use the difference in thermal energy levels between warm surface water and colder deep water to generate electricity, for example. The available energy level difference is low and so these proposals rely on the use of very large quantities of warm and cold water, and call for the necessary large volumes of deep ocean cold water to be brought to the water surface through very large vertical ducts of great length. The rate of water flow through the ducts, called "upwelling ducts", would be low, and thus the pressure differential across the walls of the duct would be correspondingly low, especially if the water in the duct is not carried far above the ocean surface before being discharged from the duct.
Any structure which extends vertically for any significant distance in the ocean will encounter at least one ocean current. Currents impose drag forces upon such structure. The larger the structure, the greater its profile (effective area) presented to the current, and therefore the greater the drag forces which a given current will impose on the structure. These drag forces impose bending loads on the structure, and the longer the structure, the more severe the resulting bending stresses. The problem of ocean current drag forces and of the bending stresses produced thereby is a significant difficulty in the offshore drilling industry in regard to the riser pipes which are used to conduct drilling mud during the drilling of a subsea oil or gas well from the well bore to the surface drilling platform. Current-induced bending stresses are so great in these riser pipes that the pipes must be made very heavy, i.e., with thick walls, to enable them to withstand the bending stresses. This problem is such that, because of it, offshore drilling operations today are effectively limited to water depths of about 1000 feet or so. This limit is imposed principally by the riser pipes.
In commonly-owned copending patent application Ser. No. 886,904 filed Mar. 15, 1978, there is described an OTEC upwelling pipe composed of individual lengths of synthetic pipe interconnected by flexible couplings. The pipe assembly is disposed about an elongate tensile core element which carries the weight of the entire pipe assembly. As described in commonly-owned copending application Ser. No. 886,907 filed Mar. 15, 1978, the tensile core of the pipe assembly can also carry the weight of a stabilizing mass suspended below the lower open end of the upweling duct as such.
These arrangements meaningfully address the problem of bending stresses induced in an upwelling pipe due to ocean current drag forces.
It has been proposed to make an OTEC upwelling pipe flexible along its length, as by defining the pipe of polyethylene. This proposal addresses the problem of stresses developed in the pipe by reason of current action on the pipe and, to a lesser extent, the problem of stresses applied to the pipe, especially at its upper end, by motions of the floating structure to which the pipe is connected in use. It has also been proposed to connect the upper end of an OTEC upwelling pipe, even one made of polyethylene, to the surface-floating structure by a connection which isolates angular motion, as in pitch and roll, of the floating structure from the pipe. These proposals are described in concurrently filed applications Ser. No. 935,673, and Ser. No. 935,591, respectively.
Upon consideration of the factors reviewed above, it is apparent that an OTEC facility poses several problems which are focused upon the connection of the upwelling pipe to the surface floating structure. The use of an angular-motion isolating mechanism in the connection does not deal with all possible motions of the floating structure in response to waves; the structure can surge, sway and heave, and such motions, if not properly dealt with, can produce high bending and shear loads in the upwelling pipe, principally at its upper end where such loads are most troublesome. Also, the connection must be strong and reliable. Further, the presence of the upwelling pipe, when connected to the floating structure, effectively precludes the floating structure from moving from its assigned station in the event of an emergency, such as a severe storm, or for needed maintenance, for example.
It is thus seen that a need exists for a strong and reliable connection arrangement between the upper end of an OTEC upwelling or riser pipe, for example, and a surface floating structure. For maximum effectiveness and utility, the connection should be useful with an angular-motion isolating mechanism between the pipe and the structure. It should also meaningfully assist in preventing undue bending and shear loads in the pipe derived from linear motions of the structure in waves. It should make possible the easy connection of the pipe to the floating structure, and also permit the pipe to be quickly and reliable released from the structure in the event of an emergency.